Just two months after finishing its first phase, the Mammoth Springs upscale apartment development in Sussex is about to start construction on two more buildings.

The $21 million Mammoth Springs is being developed on 10 acres south of Main St. and west of Waukesha Ave./Highway 74, on the site of a former cannery that closed in 1996 and was later demolished.

The three-story apartment buildings, a commercial building for the corner and underground parking are among the features that give Mammoth Springs a denser, more urban style — a first for this Waukesha County community.

A pair of 30-unit apartment buildings opened May 1 and are now 87% occupied, developer Arthur Sawall said. While that's a fast lease-up rate, he had hoped to be at 95% by now.

"I like to push things," said Sawall, who founded Brookfield-based ECT International Inc. Sawall sold ECT in 2008 to software firm Bentley Systems Inc., based in Exton, Pa., and later served as a Bentley vice president.

With the first two buildings largely leased up, Sawall plans to begin construction this month on the next phase of two more 30-unit buildings. Those buildings will be completed around May, he said.

The one- and two-bedroom apartments have monthly rents ranging from about $950 to $1,300. The plan for Mammoth Springs also calls for later developing a fifth 30-unit apartment building.

The commercial building has attracted interest from prospective tenants, but those retailers are still waiting for the economy to improve before committing to the project, Sawall said. So construction on that building remains on hold for another year or two, he said.

Sawall also has developed a walking path through the development site, which has a 3-acre quarry pond and links to Waukesha County's Bugline Recreational Trail.

Meanwhile, Sawall has dropped a tentative plan to expand the mixed-use development north of Main St. That would involve buying the Sussex Inn and two other properties, and demolishing those buildings to develop more apartments and commercial space.

"It didn't make economic sense," he said.

Financing for Mammoth Spring includes $2.8 million in village funds provided through a tax incremental financing district. The new property taxes generated by Mammoth Springs will pay back that amount.